bug#12401: scroll-preserve-screen-position broken with font height faces

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Le Wang <l26wang@gmail.com>,  12401@debbugs.gnu.org
> Date: Mon, 10 Sep 2012 09:26:42 -0400
> 
> > scroll-preserve-screen-position?  Is it just that doing N C-v's
> > followed by N M-v's brings point to the same location where it was
> > before this sequence of 2N commands?
> 
> I think this should be the main point, yes.

For this to work, we would need to make sure that C-v followed by M-v
end up with a window that has the same window-start as the one before
C-v.  But this is impossible to ensure with the way we compute
window-start, because the move_it family of functions used for that
can only move forward; therefore moving back is necessarily
implemented differently, and the results differ when variable-size
fonts are present.

IOW, this is a limitation of the current design of the
window-scrolling functions that can only be lifted by redesigning.